Video telephony (VT) involves the real-time communication of packets carrying audio and video data. A VT device includes a video encoder that obtains video from a video capture device, such as a video camera or video archive, and generates video packets. Similarly, an audio encoder in a VT device obtains audio from an audio capture device, such as a microphone or speech synthesizer, and generates audio packets. The video packets and audio packets are placed in a radio link protocol (RLP) queue. A medium access control (MAC) layer module generates medium access control (MAC) layer packets from the contents of the RLP queue. The MAC layer packets are converted to physical (PHY) layer packets for transmission across a communication channel to another VT device.
In mobile VT applications, a VT device receives the physical layer packets via a wireless forward link (FL) (or “downlink”) from a base station to the VT device as a wireless terminal. A VT device transmits the PHY layer packets via a wireless reverse link (RL) (or “uplink”) to a base station. Each VT device includes PHY and MAC layers to convert the received PHY and MAC layer packets and reassemble the packet payloads into audio packets and video packets. A video decoder within the VT device decodes the video data for presentation to a user via a display device. An audio decoder within the VT device decodes the audio data for output via an audio speaker.
Mobile VT in a wireless environment can be challenging. The data rate over the wireless channel is limited and varies with time. For example, in a CDMA2000 1x EV-DO Release 0 network, the data rate may vary due to channel conditions within a wireless coverage area or traffic congestion among multiple VT users. In addition, when the data rate drops to zero, e.g., when there is no data to send, recovery to a reasonable data rate may require time. As a result, mobile VT can be susceptible to undesirable video and audio delay, which undermines the ability to carry on smooth video conferencing in real-time.